Isolation trenches are used in SOI silicon wafers for isolating different components (e.g. transistors) in smart power ICs or entire areas having different potentials. In this case the isolation trench can encompass the component to be isolated or the area to be isolated in circular shape, cf. in this respect U.S. Pat. No. 5,734,192 or U.S. Pat. No. 6,394,638 B1.
U.S. Pat. No. 5,283,461 describes a trench configuration wherein the components to be isolated are separated by a grid of isolation trenches. For a variety of applications it is advantageous to dope the trench sidewalls. The sidewall doping can be effected for example by depositing a doped layer followed by a subsequent annealing step in which the doping atoms are diffused from the doped layer into the trench sidewalls. This is described in U.S. Pat. No. 4,676,847. It is a characterizing feature of this method that all trench sidewalls in all trenches receive the same doping.
Also doping the trench sidewalls by means of ion implantation effected at a proper angle, as described in U.S. Pat. Nos. 5,013,673, 5,047,359 and 6,509,240, produces a uniform doping of all trench sidewalls.
FIG. 1 illustrates the state of the art and shows a schematic cross section of a silicon on insulator (SOI) silicon wafer consisting of a substrate wafer 10, a buried isolating layer 12, typically made of SiO2, and an active layer 14. The isolation trench 16 consists of an isolating layer 17 and a fill layer 18.
FIG. 2 shows the structure with a buried doped layer 20 at the bottom of the active layer 14 and with a sidewall doping 22, i.e. a doping of the silicon end faces bordering the trench.
Different methods and procedures are known for differently doping the sidewalls of isolation trenches by means of diffusion.
U.S. Pat. No. 5,057,443 discloses a method wherein at first a wide recess is produced by anisotropic etching. The bottom and the two sidewalls are doped by means of gas phase doping. By means of conformal deposition of an isolating layer (e.g. silicon dioxide) the bottom and the sidewalls of the wide recess are lined. The isolating layer on the bottom of the recess is subsequently etched off with the isolating layer remaining on the sidewalls. The recess is filled again with single crystal silicon with a certain doping by means of selective epitaxy. As a result isolation trenches made of isolating material (remaining isolating layer on the sidewalls) are obtained, wherein the adjacent silicon can be doped on the one side (sidewalls of the wide recess) differently from the other side (silicon layer produced by epitaxy). However, this method will not be effective in SOI wafers since there the buried oxide cannot be used as a nucleus for epitaxial growth. It is also impossible to produce isolation trenches each having the same doping on both sides (i.e. the left and the right trench sidewall).
U.S. Pat. No. 4,756,793 describes a method which allows producing isolation trenches having different sidewall dopings. However, the method described requires a complex filling of the trenches with a fill layer, covering of a part of the trenches with a second masking layer, removing the fill layer in the open portions of the masking layer, and doping the sidewalls by implantation performed at a certain angle. By repeating the described method isolation trenches could be produced which have n-conductive sidewalls on the left and right side (or front and back side, respectively), as well as isolation trenches which are surrounded by p-conductive sidewalls on all sides. The necessity of filling and opening the trenches several times causes extensive costs and is a severe disadvantage when deep isolation trenches (with a high aspect ratio) are to be produced, since the trenches have to be filled and etched open several times.
Methods producing isolation trenches having different sidewall doping in vertical direction are described in U.S. Pat. No. 7,112,505 B2 (here with oxygen implantation which can of course be used for doping on principle), in U.S. Pat. No. 6,929,998 B2, in U.S. Pat. No. 6,808,987 B2 and in U.S. Pat. No. 6,833,305 B2. It is a common feature of all methods that they also produce isolation trenches which have different sidewall dopings only in a vertical direction (in a direction from the surface to the bottom). However, all the isolation trenches on the wafer are identical.
Another method for producing different isolation trenches is described in U.S. Pat. No. 6,486,039 B2. Here isolating layers of different thickness are produced by means of an additional masking layer (photomask or oxide hard mask). In principle the described method could be used for producing different sidewall dopings. However, there remains the disadvantage that in addition to the photo mask for producing the isolation trenches an additional mask for producing the different doping regions is necessary. Furthermore, the described method in this simple form can be used only for flat isolation trenches (with a low aspect ratio). For deeper trenches there is a problem with exposing, developing and removing the photoresist in the deep trenches.
Also the method described in U.S. Pat. No. 7,029,997 B2 uses a second masking layer for masking of portion of the isolation trenches before implantation. Similar to the method described above, such a method can be used only for relatively flat trenches. In isolation trenches with a high aspect ratio the use of an additional photo mask is problematic because the masking layer has to be removed reliably also from the depths of the trenches.
From DE 10 2005 010 944 a method is known using the varying width of isolation trenches for producing a masking layer for an additional etching step without a second photo mask. In this case a narrow trench is completely filled while a wider trench remains partially open and thus can be used as a mask for an etching step.